Heat-treated PVC-plastic panel

ABSTRACT

A method of treating PVC plates as well as plates and panels manufactured by this method. The plates and panels, in particular wall, ceiling or floor panels, comprise a heat-treated carrier plate based on polyvinyl chloride with a density of, for example, 900 to 2,500 kg/m 3  and a film applied thereon. The film is a thin PVC-film and comprises a decorative pattern directly printed thereon.

RELATED APPLICATION DATA

This application is a divisional of U.S. patent application Ser. No.16/482,872 filed Aug. 1, 2919, which is a national phase ofInternational Patent Application No. PCT/EP2018/052672 filed Feb. 2,2018, which claims priority of International Application No.PCT/EP2017/052369 filed Feb. 3, 2017, all of which are herebyincorporated herein by reference.

FIELD OF THE INVENTION

The present invention concerns a method for treating PVC plates, animproved panel, in particular a wall, ceiling or floor panel, based on ahard PVC (PVC-U) carrier plate, and a method for its manufacture.

BACKGROUND

Polyvinyl chloride has long been used in the manufacture of floorcoverings. Polyvinyl chloride (PVC) is a thermoplastic material that ishard and brittle without the addition of additives. However, themechanical properties of PVC can be adjusted over a wide range by addingsuitable auxiliary materials such as plasticizers, fillers, colorants,stabilizers, etc. For this reason, the term “PVC” in this document doesnot refer to pure polyvinyl chloride, but to PVC with the additivesnormally added and necessary for practical use, unless otherwisespecified.

A typical example from the state of the art is described in thepublication DE 10 2006 058 655 A1. This publication discloses a floorpanel in the form of a multi-layer rectangular laminate with a carrierplate made of a soft polyvinyl chloride and a décor paper layer arrangedon top of the polyvinyl chloride layer. The application of a décor papersheet is technically complex and involves considerable costs.

From DE 10 2006 029 963 A1 a floor covering made of polyvinyl chlorideis known, which is coated with a hard-wearing lacquer layer in order toimprove the durability of the PVC floor covering. The lacquer layer isbased on an acrylate resin and is to be cured with radiation. The coreof this publication lies in the addition of electrically conductivesubstances to the acrylate resin to provide the finished floor coveringwith antistatic and/or electrically conductive properties.

The WO 2008/061791 A1 of the same applicant represents a furtherdevelopment of a lacquer coating. The content of this publication ishereby incorporated in its entirety by reference. The core of theimprovement of this publication lies in the fact that two liquid,different polymer layers are applied wet-on-wet to the surface of apanel, so that a partial mixing of the coating agents takes place. Thesetwo wet-on-wet coatings are then cured together, with the curedresulting coating having a hardness gradient due to partial mixing, withthe hardness of the coating decreasing with increasing depth from thesurface of the resulting coating.

A fundamental problem with PVC plates, and therefore also with panels orfloors made of PVC, is that they are not resistant to high thermalloads, such as solar radiation. For example, PVC plates warp or bendwhen exposed to direct sunlight. This can occur when the tiles are usedas flooring and are installed close to large window areas, such as in awinter garden or the like. Other thermal influences, such as radiators,can also lead to warpage and curvature of PVC plates, so that their usein homes, for example, is subject to certain restrictions.

In the light of these known PVC plates and their disadvantages, thepresent invention aims to provide a method for the treatment of PVCplates to improve the resistance of known plates. In particular, it isthe task of the invention to provide a method for the treatment of PVCplates which can increase the resistance of the plates to solarradiation and thermal stress. A further task is to provide an improvedpanel, in particular a wall, ceiling or floor panel, comprising asupport plate of (in particular) hard PVC. Another task is to providesuch a panel that has improved durability and high-quality decorativepatterns.

These and other tasks, which are still mentioned when reading thisdescription or can be identified by an expert, can be solved by a methodfor the treatment of a PVC plate according to the following description,a panel as described below, and a method for its manufacture asdescribed below.

SUMMARY OF THE INVENTION

The present invention concerns a method of treating a polyvinyl chloride(PVC) plate comprising the following steps in the given order: providinga PVC plate; heating the PVC plate in an oven to at least 70° C.; andcooling the heated plate. The method therefore concerns the heattreatment of PVC plates. Surprisingly, it has been found that such aheat treatment leads to a significant improvement in the resistance ofknown PVC plates, particularly to heat and/or solar radiation. It isassumed that the heating of the plates up to and preferably above theglass transition temperature (glass temperature) leads to a reduction inthe internal stresses of the plate. For this purpose, the plates shouldbe heated to at least 70° C., as this temperature is close to the glasstransition temperature of conventional PVC plates, especially hard PVC.Although the physical and chemical relationships are not clear, it issuspected that heating and subsequent cooling leads to a reorientationof the macromolecules, which leads to a reduction in internal stresses,or ultimately to an improvement in the plate's resistance. Inparticular, the treatment of the PVC plate according to the inventioncan effectively reduce or prevent distortion of the plate or theoccurrence of curvatures when the PVC plate is exposed to strong directsunlight, for example. The treatment according to the present inventioncan be carried out on otherwise untreated PVC plates, such as thosecoming out of the extruder. However, it can also be applied subsequentlyto panels which have PVC plates as support plates and which may haveadditional layers. Examples of such panels are described in more detailin this Disclosure.

Preferably the plate is heated to at least its glass transitiontemperature and especially preferably to at least 75° C., morepreferably to at least 79° C., more preferably to at least 81° C., evenmore preferably to at least 85° C. The glass transition temperature ofPVC is about 80° C., but depending on the additives used, it can also beslightly lower or higher. For the purposes of this invention, it isadvantageous if the glass transition temperature of the PVC used isslightly exceeded, i.e., by a few degrees. The PVC plate shouldpreferably reach the desired temperature completely, i.e., the core ofthe plate should also preferably have the desired minimum temperature.Due to the low thermal conductivity of PVC, however, it may takerelatively long for the core, i.e., the innermost layer, of the PVCplate to reach the desired temperature. It is therefore preferred to usethe thermal energy during the extrusion of the plate and not to allowthe plate to cool completely after extrusion. Instead, the plate isallowed to cool to only 40-50° C. and then heated to above glasstemperature in an oven. The entire plate volume should preferably beevenly heated. In this way, particularly dimensionally stable plates areachieved.

Preferably the plate is kept at the minimum temperature for at least 3minutes, more preferably at least 5 minutes and most preferably at least10 minutes. It has been shown that the internal stresses of the plate,or the intended and inventive effect, can be most safely achieved if theplate is kept at the desired minimum temperature for some time beforebeing cooled down again.

Furthermore, the plate is preferably heated with a heating gradient of2° C./min to 20° C./min on average; preferably of 4° C./min to 15°C./min on average; more preferably of 6° C./min to 12° C./min on averageand most preferably of 7° C./min to 10° C./min on average. The thermalconductivity of PVC is relatively low, so that slow heating isadvantageous, as otherwise the outermost parts of the plate may beconsiderably above the desired minimum temperature, while the inside ofthe plate is still relatively cold. The slow heating in the given areasthus causes a uniform heating and thus a more uniform reorientation orrealignment of the molecules (as far as this is understood, the exacteffects of the heating on the orientation of the molecules are not yetfully known). It is assumed that this uniformity is decisive for theparticularly good properties of such slowly heated plates. At a heatinggradient of, for example, 1° C./min, the plate would heat up by approx.60° C. in one hour. The heating at the beginning may be faster thantowards the end of the process due to the high temperature differencebetween the still cold plate and the oven temperature, depending on theoven used and the type of heating. The heating gradient can therefore begreater at the beginning of the heating process than towards the end ofthe process, although this effect can be minimized by suitable ovenselection or oven control. The data on the gradient are therefore to beunderstood here as average gradients, in the sense that a gradient ofe.g., 2° C./min means that it takes approx. 40 minutes for a plate to beheated from 20° C. to 100° C. Again, it is sufficient that a large partof the plate's mass has been brought to the desired temperature, such asabout 90% of the mass, although it is basically desirable that the plateis completely heated.

The cooling process is just as important as heating the plate. In thecase of rapid and uneven cooling (e.g., shock cooling in a water bath atroom temperature), only a slight or no improvement in stability can beobserved despite previous heating. In order to achieve an improvement,the plate should be cooled down slowly and evenly to at least 50-40° C.Slow cooling, quite to room temperature, is advantageous. However, aclear improvement of the stability is to be registered also if the plateis driven into a water bath after reaching a temperature of 50° C.,however, the water bath should have a water temperature of not less than35-25° C. (depending on the initial temperature of the plate to bedipped; the temperature difference between plate and water bath shouldnot be too large).

The plate is also preferably cooled down in a controlled manner,preferably in a cooling oven, and cooled uniformly over the surface to aminimum temperature in the plate core of 30-50° C. with a coolinggradient of 3° C./min to 20° C./min on average; preferably 5° C./min to15° C./min on average; more preferably 7° C./min to 12° C./min onaverage and most preferably 8° C./min to 10° C./min on average. Thecooling gradient should be understood in the same way as the heatinggradient, i.e., as an average value until the plate has reached a coretemperature of approximately 30 to 50° C., for example only 10 degreeswarmer than the environment. The slow and controlled cooling leads toparticularly resistant plates. It is assumed that the slow cooling leadsto a good fixation of the orientation of the molecules.

Preferably the PVC plate is fed through the oven on a conveyor and theoven is a continuous oven. Heating and/or cooling in such a continuousmethod is particularly economical, since different heat zones withdifferent temperatures can be realized in a continuous oven. Thus, forexample, the temperature in the oven can be continuously increased ordecreased in the transport direction through the oven, whereby uniformgradients for heating or cooling can be realized.

Preferably, cooling of the heated plate takes place in a continuousoven. Here, too, it is advantageous to provide different temperatureranges along the transport direction of the plates through the oven, sothat the cooling gradient is kept as uniform as possible, for example bykeeping the temperature difference between the plate and the environmentof the oven approximately constant. The temperature inside the ovendecreases advantageously along the transport direction of the platethrough the oven (cooling oven). When the plate leaves the oven, itpreferentially has a temperature similar to the ambient temperature,e.g., only 10° C. or 20° C. above the ambient temperature.

According to the present invention, a panel comprising a polyvinylchloride plate (hereinafter referred to synonymously as plate andsupport plate in the description of the plate), preferably treated asdescribed above, is also provided. A film is attached to the plate,wherein the film is a PVC film having a thickness of 0.04 to 0.2 mm andcomprising a decorative pattern directly printed thereon, and a curedpolymer layer is provided over the PVC film. The polymer layer isgenerally preferably based on a polymerizable resin, in particular anacrylate resin. In general, the cured polymer layer prefers a hardnessgradient according to the WO 2008/061791 of the same applicant describedat the beginning. Heat treatment of PVC plates is preferably carried outafter application of the film (preferably PVC film). The decor printingand the application of further layers is preferably carried out afterthe heat treatment of the PVC plate provided with the film.

The plate of polyvinyl chloride consists in general preferably of hardPVC (also referred to as PVC-U), i.e., it essentially contains no or noplasticizers. This applies to all processes and products describedherein.

In contrast to the above-mentioned state of the art DE 10 2006 058 655A1, the present invention provides a panel that does not require aseparate déor paper, as the decorative pattern is printed directly ontothe PVC film. The PVC film has the advantage that it is possible todispense with expensive pre-treatment of the carrier plate (althoughpre-treatment is of course possible, should this be desired). Inparticular, there is no need to grind the surface of the carrier plate,and the application of fillers and primers, which is typically necessaryin the state of the art, can also be advantageously omitted if a PVCfilm is used in the thickness in accordance with the invention.

In a preferred embodiment, the polymer layer has a hardness gradient, sothat the hardness of the polymer layer decreases essentiallycontinuously with increasing depth as seen from the surface of thepolymer layer. Such a layer with a hardness gradient is advantageous incombination with the relatively soft PVC film and, among other things,achieves good sound insulation.

The printing ink used for printing the decorative pattern is solventbased and preferably a UV printing ink. Such printing inks slightlydissolve the surface of the PVC film, resulting in a firm anchorage ofthe printing ink on the film. The use of UV inks also results in verygood cross-linking of the ink with the surface of the PVC film. UV inksare therefore particularly preferred because they contain reactivesolvents that are chemically incorporated into the later network, suchas N-vinylcaprolactam.

Preferably a polymerizable printing ink (die) is therefore used forprinting the decorative pattern, in particular based on polymerizableacrylic resins and/or N-vinylcaprolactam (a liquid reactive diluent) asoffered by the company BASF. The applicants have surprisingly found thatimproved adhesion properties of the layer system can be achieved byusing polymerizable printing inks instead of the common water-basedemulsion inks. This applies in particular to the preferred applicationwith a polymer layer, especially with a hardness gradient. Suitableweight proportions for the amount of polymerizable acrylate andN-vinylcaprolactam in the ink have been found to be 2 to 50, morepreferred 5 to 40 and most preferred 10 to 30%. These values refer tothe sum of acrylate and N-vinylcaprolactam. The proportion by weight ofN-vinylcaprolactam in the printing ink, for example, is 3 to 12%.

The positive effect is particularly noticeable if the printing ink ofthe décor layer (i.e., the decorative pattern) and the polymer layer arecured or polymerized together (if the décor layer is printed directlyon, as in digital printing, the décor layer virtually consists of theprinting ink). Curing of a polymer layer or a polymerizable printing ink(such as polymerizable acrylates or UV-reactive inks in general) isunderstood to mean the chemical reaction that takes place duringpolymerization. The joint curing (polymerization) of the polymerizablecomponents (acrylate systems and/or N-vinylcaprolactam) of the ink andpolymer layer results in chemical crosslinking at the interface of thetwo layers, which is assumed to be responsible for the improved adhesionof the layers.

Polymerizable components, which are preferably used, include acrylatesas their main components, in particular acrylate monomers, oligomers andoptional photoinitiators, but also N-vinylcaprolactam as a liquidreactive diluent. N-vinylcaprolactam can be added to the printing ink asa thinner in addition to the acrylates and polymerizes together withthese. Alternatively, it is also possible to dispense with the acrylatesand provide a correspondingly larger amount of N-vinylcaprolactam, sincethe N-vinylcaprolactam itself can be polymerized. Details on this areknown to the expert from the German publication DE 197 02 476 A1. Inpreferred embodiments, the polymerizable components thereforeessentially consist of N-vinylcaprolactam. The photoinitiators effect apolymerisation of the monomers or oligomers under the influence ofradiation, whereby the printing ink cures quickly.

Preferably the PVC film has a thickness from 0.05 to 0.15 mm, morepreferably from 0.06 to 0.095 mm. Such thicknesses can be processed verywell and especially applied with a calender. For example, the PVC filmcan be applied directly using a heated calender so that the film isthermally bonded/fused to the carrier plate. The use of an additionaladhesive to attach the film to the carrier plate is then not necessary,although this is possible alternatively or additionally of course.

Preferably the polyvinyl chloride plate has a density of 900 to 2,500kg/m³, preferably 1,000 to 2,200 kg/m³, more preferably 1,300 to 1,950kg/m³ and most preferably 1,350 to 1,500 kg/m³. Such densities result invery robust and resistant plates, which are particularly advantageouswhen panels are used as floor coverings. In addition, these plates allowthe incorporation of locking or coupling elements at the side edges ofthe plate, e.g., in order to be able to connect several similar plateswith each other in a form-fitting manner.

A thickness (strength) for the PVC plate (or carrier plate) between 3and 20 mm, preferably between 4 and 15 mm, more preferably between 3 and12 mm and most preferably between 4 and 10 mm has proved to beadvantageous. It has been found that these areas provide sufficientstability for the thickness of the carrier plate during themanufacturing process and also provide sufficient impact soundabsorption (when used as flooring) and dimensional stability of thefinished panel.

In a preferred embodiment, a layer comprising a UV primer is provided onthe PVC film. This layer preferably has a surface mass of 1 to 20 g/m²,more preferably 2 to 15 g/m², and most preferably 2 to 5 g/m².Dipropylene glycol diacrylate has proven to be a suitable material,e.g., in an amount of 2 g/m² it has a good effect. The primer ispreferably applied to the printing ink and thus improves the adhesionbetween the printing ink, the substrate and the polymer layer. It hasbeen shown that especially in areas with little ink the adhesion isimproved by using the primer.

The PVC plate is generally preferably an extruded PVC plate.

In a preferred embodiment, abrasion-resistant particles, especiallycorundum particles, with an average diameter of 10 to 100 μm, preferably20 to 80 μm, more preferably 25 to 70 μm and especially 30 to 60 μm areembedded in the polymer layer. The abrasion-resistant particles make itpossible to considerably increase the lifetime of a panel according tothe invention.

In a preferred embodiment, the decorative pattern, which is applieddirectly to the PVC film using a digital printing process, representsthe only décor layer of the panel according to the invention, i.e., nofurther décor papers or décor films are provided. There is therefore noneed to provide a separate décor paper or the like, which leads to aconsiderable reduction in costs and simplification of the manufacturingprocess for a panel according to the invention.

As mentioned at the beginning, according to a preferred embodiment ofthe invention, the printing ink of the décor layer (of the decorativepattern) is cured (polymerized) together with the polymer layer appliedover it, preferably by irradiating it together. This results in apartial chemical crosslinking of the polymers used at the interfacebetween the printing ink and the polymer layer(s) applied above it. Ithas been shown that a particularly good adhesion of the polymer layer tothe carrier plate can be achieved.

According to this invention, a method of manufacturing a panel, inparticular a wall, ceiling or floor panel, is also provided.

The first step in this method is to provide a polyvinyl chloride plate(carrier plate), which has preferably been treated with one of themethods described above (heat treatment/temperature treatment). Inaddition, the panel preferably has a density of 900 to 2,500 kg/m³.Alternatively, an untreated PVC plate can also be provided, and the heattreatment described above can only be carried out after some or all ofthe other layers have been applied to the plate. A PVC film with athickness of 0.04 to 0.2 mm, preferably by means of a calender, isapplied to this carrier plate and then the heat treatment is preferablycarried out. After the heat treatment, a decorative pattern is printedon the PVC film.

After printing the decorative pattern on the PVC film, a liquid firstpolymer layer is applied to the primer layer. In a further step, thepolymer layer is cured, preferably together with the printing ink.Optionally, at least a second polymer layer can be applied in liquidform to the still wet first polymer layer so that the coating agents arepartially mixed. The curing of the printing ink and the polymer layer(s)can take place in one method step but also successively in two separatemethod steps. Hardening or curing of a polymer layer is understood to bethe chemical reaction that takes place during polymerization. Adistinction must be made between this and the drying of such layers, inwhich only the water content is reduced or removed.

Preferably the first and second polymer layers are applied such that thecured polymer layer has a hardness gradient, wherein the hardness of thecoating decreases with increasing depth as seen from the surface of theresulting coating. This process is described in more detail in the WO2008/061791 A1 mentioned at the beginning, so that no further detailsare given here.

Preferably, a heated calender is used in such a way that the PVC film isthermally welded to the carrier layer. In this way, the film can beapplied and fastened easily and safely in a single step.

Furthermore, the directly printed decorative pattern preferably imitatesa wood, stone or tile surface. The PVC film is generally preferredunicoloured or white. This provides a good background colour for mostcommercially used decors.

In the following, the advantages will be explained in more detail usingtwo non-restrictive examples:

Reference Example: “Panel with Conventional Indirect Gravure Printing”.

A PVC carrier plate with a thickness of 4 mm is first provided with aprimer based on a commercially available aqueous acrylic dispersion viaa roller applicator. In the next step, the PVC carrier plate is smoothedby a roller applicator using a smoothing compound based on a highlyfilled aqueous acrylate dispersion. Subsequently, a primer based on anaqueous acrylate dispersion mixed with fillers and colour pigments isapplied by casting. After each of these coating steps, intermediatedrying is carried out at temperatures between 80 and 200° C. The PVCcarrier plates treated in this way are supplied to a printing machineessentially consisting of an engraving roller and a rubber roller fortransferring the print image from the engraving roller (engravingcylinder) to the plate. The print image is produced by three downstreamprinting units, whereby each printing unit applies its own printing inkconsisting of ink pigments and an aqueous acrylic dispersion. Forexample, if a dark walnut wood is imitated, 5 g/m² of printing ink isapplied. A commercial UV primer is then applied to the ink layer via aroller applicator. Finally, the polymer layer is applied as described inWO 2008/061791 to produce a polymer layer with a hardness gradient.

Example: “Panel Where the Pattern is Printed on a PVC Film”.

A 6 mm thick PVC carrier plate made of hard PVC is used and a PVC filmwith a thickness of 0.03 mm is applied to it using a heated calender, sothat the film is thermally welded to the carrier plate. The plateproduced in this way is then subjected to the heat treatment describedabove after intercooling to 30-50° C., preferably. On the attached PVCfilm, the same décor image as in example 1 is produced using a digitalprinter. However, a solvent-based, UV-curing digital printing ink isused. To produce the printed image, a quantity of ink of about 2 g/m² isrequired. The ink is first fixed with 150 mJ/cm² (mercury). Then 2 g/m²of a first UV-curing layer is applied, which mainly contains dipropyleneglycol diacrylate. On this non-irradiated layer a double bond containingoligomer mixed with photoinitiators is applied. The compound is thenirradiated with a UV radiation source and the polymerizable componentsare cured. The resulting polymer layer comprises the printing ink andall layers above it.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is explained in more detail using thefigures, wherein:

FIG. 1 a schematically shows a device for the heat treatment of a PVCplate;

FIG. 1 b schematically shows the temperature curve in the device;

FIG. 2 shows a schematic diagram of a plate 10 with a carrier plate 12made of polyvinyl chloride;

FIG. 3 shows a schematic view of a coating device; and

FIG. 4 schematically shows an experimental setup for the comparison oftreated and untreated panels.

DETAILED DESCRIPTION

FIG. 1 schematically shows a device for the heat treatment of a PVCplate. The device essentially consists of an oven 1, which in theexample shown is a continuous oven. The oven is equipped with a conveyor2 which moves the parts to be treated through the oven in the directionof arrow 3. The reference sign 12 indicates PVC plates which areconveyed through the oven from left to right. Reference number 10indicates finished panels which have already been coated or providedwith films as described below. Such coated panels 10 can also besubjected to a heat treatment in order to treat the respective carrierplates 12 of the panels 10 accordingly. The oven 1 comprises seven zonesO1 to O7 in which different temperatures are present. Of course theshown device is only exemplary and also devices with more or less zonesas well as devices with completely different temperature profiles aspresented herein are possible. The plates 12 or panels 10 enter thefirst zone O1 of the oven. The temperature within the first zone O1rises continuously in the conveying direction of the oven.

The temperature course or the temperature profile within the individualzones of the oven is shown in FIG. 1 b . The temperature is shown on thevertical axis and the horizontal axis corresponds to the course alongthe length of the oven (L_(Oven)). The dotted lines indicate thetransition between the different zones O1 to O7. In the example shown,the temperature within zone O1 rises continuously with a relatively lowrate of increase. The reason for this is that the panels or plates havea relatively low temperature when entering the oven, such as roomtemperature, and therefore even relatively low oven temperatures lead torapid heating of the plates. This is because the speed at which the bodyis heated depends fundamentally on the temperature difference betweenthe body and the surrounding air, for example. As the skilled personknows, large temperature differences lead to a greater transfer of heatenergy and thus to a faster heating of the body. By a suitable selectionof the temperature profile in the conveying direction of the oven, thetemporal course of the heating of the plates can be controlled. It isdesirable that the heating should be as homogeneous as possible, i.e.,the heating or cooling gradient should be as constant as possible duringtreatment.

In zone O2, the temperature rises relatively more in the conveyingdirection than in zone O1. In zone O3 the temperature is kept constant,and the plates or panels are kept at the desired target temperature fora few minutes during transport through zone O3. In zone O4, thetemperature in the oven is slowly lowered in the conveying direction, ascan be seen from the relatively flat temperature course in FIG. 2 b . Inthe following zones O5 to O7 the temperature then drops further andfurther, so that the plates or panels are slowly cooled down to atemperature close to the room temperature (e.g., 30 or 35° C.). Thecooling phase is advantageous, as shown, longer than the heating phase,i.e., cooling should preferably take place relatively slowly. After theplates have left the oven, they can be stored or further processed asrequired.

FIG. 2 shows a panel 10 that is provided with several films or layersand can be used, for example, as a floor panel. The panel 10 comprises aplate (carrier plate) made of PVC 12, which has tongue and grooveconnections at its respective edges, which allow individual panels 10 tobe connected with each other. The carrier plate consists of an extrudedhard PVC (PVC-U) and can, for example, be heat-treated using one of themethods described herein.

Above the (carrier) plate 12 a PVC film 17 is arranged. A decorativepattern (décor layer) 18 is printed on the top side of film 17,preferably by means of a digital printing process. This decorativepattern can be any pattern, depending on the application. Above the PVCfilm and the décor layer, a UV-curable polymer layer system 19 isprovided. The illustration is not true to scale, and the layers areshown here at a distance from each other not present in the real productin order to make them more clearly visible. In particular, the plate 12is considerably thicker than the layers applied to it, namely in therange of several mm, whereas the layers applied to it represent only afraction of a mm in total.

In the following, FIG. 3 is used as an example to describe themanufacture of a panel according to the invention or the method requiredfor this. FIG. 3 schematically shows a coating device for the coating ofplates 12 or for the production of panels 10. The plates 12 consist ofhard PVC with a thickness of 4-8 mm and were first subjected to the heattreatment described herein. Alternatively, the heat treatment describedherein can also be carried out subsequently on the finished panel 10 oran intermediate product. The plates 12 are guided by a roller conveyor21 through the various stations of the coating device. The coatingstations shown are not to be understood conclusively, but serve only asexamples to explain the method according to the invention and are shownpurely schematically. In front of, behind and between the stationsshown, further processing stations may be provided, such as furtherdrying stations, stations for applying primers, stations for applyingfillers, etc. The first station 30 is intended to be a calender unitused to apply the PVC film 17 to the top of the plates 12. The film isunwound from a supply roll 31 and attached to the top of the plates 12by a heated calender roll 32. The film is cut to size using suitablecutting means known to a person skilled in the art (not shown).

In Station 60, a decorative décor, in particular a real wood décor, isprinted on PVC film 17 using digital printing. After printing, a polymerlayer is applied in the coating station 70. The polymer layer is appliedwith a hardness gradient, so that the hardness of the polymer layerdecreases essentially continuously with increasing depth as seen fromthe surface of the polymer layer. For this purpose, a first polymerlayer based on a polymerizable acrylate system is applied in a firstcoating unit 71. A further wet-on-wet polymer layer is applied to thisfirst polymer layer in Station 72. The second polymer layer, forexample, has a higher double bond content, as described in detail in theabove-mentioned application on the hardness gradient. The two polymerlayers are applied wet-on-wet in stations 71 and 72, so that partialmixing occurs at the interface of the two layers. In Station 73, the twopolymer layers are cured together under the influence of UV radiation.

Station 60 is preferably a digital printing station and uses a printingink based on a polymerizable acrylate. In this case, it is preferablethat no curing of the ink takes place between stations 60 and 70, but atmost an intermediate drying step during which some moisture is removedfrom the polymerizable acrylate of the ink. In the curing station 73,the printing ink and the first and second polymer layers are then curedtogether, resulting in a particularly resistant surface.

Comparison of Standard PVC and Heat-Treated PVC

The effect of heat treatment according to the invention was investigatedexperimentally. For this purpose, a PVC carrier plate with a density of2,050 kg/m³ was extruded on a twin-screw extruder. This plate was thenprovided with a decorative high-performance layer and processed intofloor panels 510. Panels manufactured in this way were laid to a testarea of approx. 2×4 m (“standard PVC”) as sketched in FIG. 4 . As acomparison, after cooling to approx. 40° C., an identically extrudedplate was heated again to 85° C. in an oven in accordance with theinvention and then slowly cooled again (“heat-treated PVC”). Afterwardsthe same coating, panel production and laying to a second test area asin the case of the untreated panels 510 was carried out.

Both test areas were irradiated in an area of approx. 1 m² from abovewith four IR radiators 501 (as a simulation of solar radiation throughdeep-drawn windows, e.g., in a winter garden). The radiators heated thesurfaces of the floors or panels at a rate of approx. 1° C./min up to asurface temperature of approx. 80° C. During irradiation, the maximumcurvature of the panels was measured and recorded. The results of thistest are summarized in the table below:

Standard-PVC Treated PVC Start curvature after t 25 None in min: Startcurvature at 50 None temperature in ° C.: Maximum curvature 9.46 None inmm Maximum curvature at 68.6 None temperature in ° C.: T_(max) duringtest 75.6 81 in ° C. Duration of irradiation 127 124 in min

In the case of non-heat-treated PVC (“standard PVC”), the floor beneaththe 501 infrared lamps began to curve significantly after 25 minutes ata surface temperature of 50° C. The curvature occurred over a largearea, as indicated by circle 502 in FIG. 4 . The floor of treated PVCplates showed no visible curvature. The untreated floor (“Standard-PVC”)reaches its maximum curvature of 9.5 mm at a surface temperature of68.6° C. The floor of treated PVC plates showed no change at thistemperature. The test was aborted after 127 min and a surfacetemperature of 81° C.

Throughout the area investigated, the floor of treated PVC plates wasstable and showed no curvature behaviour. The surface temperaturesachieved in the test correspond to those that can be achieved inpractice when floors are exposed to direct sunlight, especially withdark decors. The problems with the standard PVC regularly led tocomplaints, so that a number of panel manufacturers point out on theirpackaging that the products are “not suitable for e.g., winter gardensand places with direct sunlight”. The plates treated according to theinvention do not show such problems.

The invention claimed is:
 1. A panel, in particular a wall, ceiling orfloor panel, comprising a polyvinyl chloride plate and a film attachedto the polyvinyl chloride plate, wherein the film is a PVC film having athickness of 0.04 to 0.2 mm and having a decorative pattern directlyprinted thereon, wherein a cured polymer layer is provided over the PVCfilm, and wherein the polymer layer comprises a hardness gradient, sothat the hardness of the polymer layer decreases essentiallycontinuously with increasing depth as seen from the surface of thepolymer layer.
 2. The panel according to claim 1, wherein the printingink used for printing the decorative pattern is solvent based.
 3. Thepanel according to claim 2, wherein the printing ink contains apolymerizable acrylate and/or N-vinylcaprolactam.
 4. The panel accordingto claim 3, wherein the printing ink for printing the decorative patterncontains a polymerizable acrylate and N-vinylcaprolactam in a weightproportion in the ink of 2 to 50%.
 5. The panel according to claim 1,wherein the printing ink and the polymer layer are cured together. 6.The panel according to claim 1, wherein the decorative pattern wasapplied by digital printing.
 7. The panel according to claim 1, whereinthe PVC film has a thickness from 0.05 to 0.15 mm.
 8. The panelaccording to claim 1, wherein the PVC plate has a thickness between 3and 20 mm.
 9. The panel according to claim 1, wherein a layer comprisinga UV primer is provided on the PVC film.
 10. The panel according toclaim 9, wherein the layer comprising a UV primer (14) has a surfacemass of 1 to 15 g/m².
 11. The panel according to claim 1, wherein thePVC plate is an extruded PVC plate.
 12. The panel according to claim 1,wherein the PVC film is glued or thermally welded to the plate.
 13. Thepanel according to claim 1, wherein abrasion-resistant particles areprovided in the polymer layer.
 14. The panel according to claim 1,wherein the polymer layer is based on one or more of the followingacrylates: 1,6-hexane dioldiacrylate, polyester acrylate, polyurethaneacrylic acid ester and dipropylene glycol diacrylate.
 15. The panelaccording to claim 1, wherein, besides the decorative pattern on the PVCfilm, no further décor layers in particular no décor papers or décorfilms are provided.
 16. The panel according to claim 1, wherein thedirectly printed decorative pattern imitates a wood, stone or tilesurface.
 17. The panel according to claim 1, wherein the polyvinylchloride plate contains no plasticizers.
 18. The panel according toclaim 1, wherein the polyvinyl chloride plate consists of hard PVC(PVC-U).
 19. The panel according to claim 1, wherein the printing inkused for printing the decorative pattern is a UV printing ink.
 20. Thepanel according to claim 13, wherein the abrasion-resistant particlescomprise an average diameter of 10 to 150 μm.